1. Field of the Invention
This invention relates in general to power distribution networks and, more particularly, to means for protecting power distribution networks from damage due to ground faults.
2. Description of the Prior Art
Networks of bus bars and cables are used to distribute electric power from a source of supply, such as a transformer, to the point where it is ultimately used by motors, lights,, or other loads. In order to provide for safe operation of these networks overcurrent protection has traditionally been supplied, using circuit interrupters such as fuses, switches, and circuit-breakers to interrupt current flow through a portion of the network when such current flow rises above a safe level. The current level at which overcurrent protective devices operate is normally set at the point at which current flow through the conductors of the network produces sufficient resistance heating to damage the conductors themselves or objects in close proximity to the conductors. Such overcurrent conditions can occur due to short circuits between the conductors or in the apparatus being powered by the network.
Another type of undesired condition which can occur in power distribution networks is a ground fault. This is produced when current flows from a network phase conductor to an object at ground potential. The fault may be either a bolted fault, that is, a direct metallic connection between the network conductor and an object at ground potential, or an arcing fault wherein current flows through an arc between the network conductor and the grounded object. Either type of fault can result in fault current which is below the level at which overcurrent protection devices are set to trip, yet which can still cause serious damage. In multiphase ungrounded systems an arcing fault can cause voltage at other points in the network to rise to values several times higher than the normal system voltage, thereby causing damage to equipment connected to the network. In addition, the heat produced by an arcing fault is often sufficient to cause fire or explosion in the facility being served by the power distribution network.
In providing ground fault protection, devices such as current monitors are used to detect ground fault currents much below the tripping level of overcurrent protective devices. These devices operate in alternating current sytems by encircling all conductors of a branch of the network. Under normal conditions, all branch current traveling away from the power source on one branch conductor must return on one or more of the other branch conductors. Thus, the net, or differential, current flow through the branch surrounded by the current monitor is zero.
When traditional ground fault protective schemes are applied to multiphase alternating current networks incorporating a neutral conductor, the difficulty of providing ground fault protection is increased. Unbalanced loads on the network can cause current to flow through the neutral conductor which can result in a spurious ground fault indication by the current monitor. This problem is intensified when a network is served by multiple sources of power, each having its neutral grounded at a separate point with a common neutral connection. In the past, providing ground fault protection on such a system has required careful routing of phase and neutral conductors, with the ultimate layout resulting in greater lengths of conductor than would be needed for systems not having ground fault protection. Other prior art ground fault schemes were further complicated with interlocking circuit-breakers and other auxiliary equipment.
It would be desirable to provide a ground fault protection system giving greater flexibility in routing of conductors while eliminating the need for complex interlocking or excessive auxiliary control equipment. It is also desirable to provide a ground fault system which will not be affected by grounding the neutral conductor of multisource networks at multiple points.